1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image that is used for developing an electrostatic latent image formed by an electrophotographic process or an electrostatic recording process with a developer, a process for producing the same, a developer for developing an electrostatic image, and a process for forming an image.
2. Description of the Related Art
A process for visualizing image information through an electrostatic latent image, such as an electrophotographic process, is being utilized in various fields. In the electrophotographic process, an electrostatic latent image is formed on a photoreceptor through charging and exposing steps, and the electrostatic latent image is developed with a developer containing a toner and then visualized through transferring and fixing steps. The developer includes a two-component developer containing a toner and a carrier, and a one-component toner using solely a magnetic developer or a nonmagnetic toner. A kneading and pulverizing process is generally used for producing a toner, in which a thermoplastic resin is melted and kneaded with a pigment, a charge controlling agent and a releasing agent, such as wax, and after cooling, the mixture is finely pulverized and classified. Inorganic or organic fine particles are sometimes added to the surface of the toner particles depending on necessity, so as to improve the flowability and the cleaning property.
In a color electrophotographic process, which is widely spread in recent years, a releasing agent, such as wax, is generally difficult to be used in order to realize gloss and transparency that are suitable for a color image, i.e., excellent transparency for obtaining an OHP image. Therefore, when a large amount of an oil is applied to a fixing roll for assisting release, sticky feeling in a complex image including an OHP image and difficulty in writing in an image with a pen often occur. In general, wax used for monochrome electrophotography, such as polyethylene, polypropylene and paraffin, is difficult to be used for forming an OHP image because it impairs transparency.
Even when the transparency is not pursued, since a toner produced by the conventional kneading and pulverizing process cannot prevent exposure of a releasing agent to the surface of the toner, problems of remarkable deterioration in flowability and filming on the developing device and the photoreceptor occur upon using as a developer.
As an ultimate solution for removing the problems, such a method for preventing exposure of a releasing agent on the surface by embedding inside the toner is proposed, in which an oily phase containing a monomer as a raw material of a resin and a colorant is dispersed in an aqueous phase and is directly polymerized to form toner particles.
An emulsion polymerization process with aggregation and melt-fusing is proposed in JP-A-63-282752 and JP-A-6-250439 as a production process of a toner, the toner shape and the surface structure of which can be controlled according to the purpose. In the process, a resin particle dispersion is formed by emulsion polymerization, and a colorant dispersion is formed by dispersing a colorant in a solvent, both of which are mixed to form aggregated bodies corresponding to the particle diameter of the toner, followed by integrating the aggregated body by fusing the resin particles under heating.
In the electrophotographic process, in order to maintain the stable performance of a toner under various types of mechanical stress, it is necessary that the exposure of a releasing agent on the toner surface is prevented, and the surface hardness and the surface smoothness of the toner are increased.
The exposure amount of a releasing agent on the toner surface is decreased in order to exhibit the stable releasing performance of the releasing agent even in the case where an oil is applied to a fixing roll and the case where a large amount of an external additive is added to the toner surface. However, in order to exhibit further the releasing performance upon fixing, it is desirable that the releasing agent is present in the vicinity of the toner surface.
It is an important problem in recent years that the color electrophotographic process involves a problem in consuming electric power. Since a color image is formed with three layers, i.e., cyan, magenta and yellow, in a high density area, the height of the toner layer becomes larger than a monochrome image, and electric power required for fixing the color image becomes larger. Accompanying the wide spread of the color electrophotographic process, the increase in consuming electric power upon fixing becomes the limiting factor of the process speed.
Therefore, a color toner that can be fixed at a lower temperature is demanded. However, when the molecular weight or the glass transition temperature of the binder resin is simply decreased, problems occur in offset at a high temperature and preservation property of an image (such as sticking of documents upon accumulating the documents or allowing a booklet to stand at a high temperature) after fixing.
In the case where a large amount of wax having a relatively low melting point is used or the glass transition point of the binder resin is decreased to prevent offset at a high temperature, when a document fixed in a duplicating machine as an original copy is supplied to an automatic copy feeding machine, a part of a toner image is adhered to a document table due to heat from the document table and friction caused by the automatic copy feeding machine, so as to cause contamination of the document table.